Residual Dust Charges in a Complex Plasma Afterglow

IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. X, NO. X, MONTH 2011

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Residual Dust Charges in a Complex Plasma Afterglow Brett Layden, L´ena¨ıc Cou¨edel, Alexander A. Samarian, and La¨ıfa Boufendi

Abstract—The trajectories of dust particles in the afterglow of a complex plasma are presented. The effect of a bias voltage applied to the electrodes on the trajectories evidences a residual charge on dust particles, and a coexistence of negatively and positively charged particles in the late afterglow. Index Terms—Afterglow, charge, complex plasma, dust cloud, dusty plasma.

I NTRODUCTION COMPLEX (dusty) plasma is an electron-ion plasma with an additional component of nanometer or micrometer-sized particles, known as dust particles.. Dusty plasmas may be formed by either the injection of particles into the plasma, or by particle growth in the plasma. Injected particles are typically micrometer sized and thus are confined to the sheath region, where the electric force balances gravity. However, dense clouds of dust particles which are light enough to fill the entire discharge chamber can be formed by particle growth in the plasma. Dust growth can be achieved either through the use of reactive gases such as silane [1] or by sputtering [2], where energetic ions from the plasma collide with a target causing material to be ejected into the plasma. Dust particles in plasmas acquire charges due to collection of electrons and ions on their surfaces. In laboratory discharges, the higher mobility of plasma electrons results in negatively charged dust particles [3]. This charge means that they will interact with electrons, ions, neighboring dust particles, and any external electric fields present. The dust charge is therefore a very important parameter in governing the dynamics of the complex plasma. If the external electromagnetic fields that maintain the ionization are switched off, the plasma decays, which means that the electrons and ions are lost over a relatively short timescale (∼milliseconds). This loss is due to diffusion of the plasma particles to the walls of the chamber, and recombination on the dust particles’ surfaces. In the late afterglow of a complex plasma, where the electron and ion densities become small, there is little change in the dust charge due to the small flux of plasma particles on their surfaces. Experiments have shown that in both microgravity [4] and onground [5] experiments, dust particles do not lose all of their initial charge in the

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B. Layden and A. A. Samarian are with the School of Physics, The University of Sydney, Sydney, N.S.W. 2006, Australia. L. Cou¨edel is with the Max Planck Institute for Extraterrestrial Physics, 85741 Garching, Germany. L. Boufendi is with the Groupe de Recherches sur l’Energ´etique des Millieux Ionis´es, Centre National de la Recherche Scientifique–Universit´e d’Orl´eans, 45067 Orl´eans Cedex 2, France. Manuscript received MONTH DAY, 2010; revised MONTH DAY, 201X.

afterglow, but keep small “residual” charges for a long time (∼minutes) after the power supplied to the discharge is turned off. In Ref. [5] the oscillations of individual dust particles due to an external alternating electric field were analyzed; a coexistence of positively, neutrally, and negatively charged dust particles was observed, with the size of the dust charge being up to a few elementary charges. Since charged dust particles experience electrostatic interactions, analyzing their motion when an external electric field is applied can give information about the charges. Experiments were performed in the PKE-Nefedov reactor, which is a RF sputtering discharge with two parallel electrodes 4.2 cm in diameter separated by 3 cm. Dust particles were grown in argon plasma with a pressure of 1.5 mbar, by sputtering of a polymer layer deposited on the electrodes. The dust particles were illuminated by a thin sheet of laser light, and a CCD camera recorded the light scattered by them. The camera was placed at an angle of 30◦ from the normal to the laser light sheet, and captured images at 500 frames per second. Three different experimental conditions were studied: these were no bias voltage, +10 V bias, and −10 V bias applied to the lower electrode after the RF field was turned off. If the dust particles keep a residual charge, then they will experience a force upward or downward depending on the sign of the charge and the direction of the electric field from the bias voltage. After the RF field was turned off, the dust cloud fell down due to gravity. In order to analyze the vertical motion of the dust particles in the afterglow, a vertical strip of the video was chosen, and an image of this strip in each frame was obtained. The strip images were then concatenated horizontally to give the intensity of scattered light as a function of height and time. Since the scattered light intensity increases with the dust particle number density, information about the dust particles’ trajectories can be inferred. The evolution of scattered light intensity between the upper electrode and the dust void, which is the region free of dust, is shown in Fig. 1. The first notable feature of these graphs is that the experiments with a bias voltage separate the trajectories of the dust particles compared with the non-biased experiment. This means that the dust particles have a larger distribution of velocities resulting from the bias voltage. In both experiments with a bias voltage, the dust particles occupy a larger proportion of the vertical axis, and the trajectories are more dispersed. Firstly, this shows that the dust particles indeed keep a residual charge, as only charged particles would have their motion affected by a bias voltage. Also, for both bias directions, there are dust particles that fall more slowly than

IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. X, NO. X, MONTH 2011

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Fig. 1. Evolution of scattered light intensity for (a) no bias voltage, (b) +10 V bias, and (c) −10 V bias on the lower electrode. The vertical axis shows the distance below the upper electrode and the color represents the intensity of scattered light, hence the dust particle number density, in arbitrary units. The distribution of dust particle charges inferred from this figure is due to the stochastic nature of charging via the collection of electrons and ions on the surfaces of the dust particles.

in the non-biased case. This implies a force acting upwards on some dust particles for both bias directions. Thus, there are both negatively and positively charged particles coexisting in the late afterglow. This is in agreement with previous measurements [5]. The motion of the dust cloud under different experimental conditions can thus give information about the residual charge on dust particles. The dust particle trajectories show the coexistence of positively and negatively charged dust particles in the late afterglow. ACKNOWLEDGMENT This work was supported by the Australian Research Council.

R EFERENCES [1] M. Cavarroc, M. Mikikian, G. Perrier, and L. Boufendi, “Single crystal silicon nanoparticles: An instability to check their synthesis,” Appl. Phys. Lett., vol. 89, no. 1, p. 013 107, Jul. 2006. [2] D. Samsonov and J. Goree, “Particle growth in a sputtering discharge,” J. Vac. Sci. Technol. A, Vac. Surf. Films, vol. 17, no. 5, pp. 2835–2840, Sep. 1999. [3] S. Vladimirov, K. Ostrikov, and A. Samarian, Physics and Applications of Complex Plasmas. London, U.K.: Imperial Press, 2005. [4] A. Ivlev et al., “Decharging of complex plasmas: First kinetic observations,” Phys. Rev. Lett., vol. 90, no. 5, p. 055 003, Feb. 2003. [5] L. Cou¨edel, M. Mikikian, L. Boufendi, and A. A. Samarian, “Residual dust charges in discharge afterglow,” Phys. Rev. E, vol. 74, no. 2, p. 026 403, Aug. 2006.